Add metrics for Memory Cache.

src/workerd/api/memory-cache.c++

@@ -240,13 +240,23 @@ SharedMemoryCache::Use::~Use() noexcept(false) {
 
 kj::Maybe<kj::Own<CacheValue>> SharedMemoryCache::Use::getWithoutFallback(
     const kj::String& key) const {
-  auto data = cache->data.lockExclusive();
+  kj::Locked<ThreadUnsafeData> data;
+  {
+    auto memoryCacheLockRecord =
+        MemoryCacheLockRecord(IoContext::current().getMetrics().getMemoryCacheObserver());
+    data = cache->data.lockExclusive();
+  }
   return cache->getWhileLocked(*data, key);
 }
 
 kj::OneOf<kj::Own<CacheValue>, kj::Promise<SharedMemoryCache::Use::GetWithFallbackOutcome>>
 SharedMemoryCache::Use::getWithFallback(const kj::String& key) const {
-  auto data = cache->data.lockExclusive();
+  kj::Locked<ThreadUnsafeData> data;
+  {
+    auto memoryCacheLockRecord =
+        MemoryCacheLockRecord(IoContext::current().getMetrics().getMemoryCacheObserver());
+    data = cache->data.lockExclusive();
+  }
   KJ_IF_SOME(existingValue, cache->getWhileLocked(*data, key)) {
     return kj::mv(existingValue);
   } else KJ_IF_SOME(existingInProgress, data->inProgress.find(key)) {

src/workerd/io/observer.h

@@ -28,6 +28,37 @@ class WebSocketObserver: public kj::Refcounted {
   virtual void receivedMessage(size_t bytes) {};
 };
 
+// Collects metrics on memory cache, like lock wait time.
+class MemoryCacheObserver: public kj::AtomicRefcounted {
+public:
+  virtual void start() {};
+  virtual void end() {};
+};
+// turn into requestobserver
+class MemoryCacheLockTiming {
+public:
+  // Unlikely we'll get to report this without exposing internals of lock
+  //virtual void waitingForLock() {}
+  virtual void start() {}
+  virtual void stop() {}
+  virtual void locked() {}
+};
+
+class MemoryCacheLockRecord {
+public:
+  explicit MemoryCacheLockRecord(kj::Maybe<kj::Own<MemoryCacheObserver>> observer)
+      : memoryCacheObserver(kj::mv(observer)) {
+    KJ_IF_SOME(o, memoryCacheObserver) o.get()->start();
+  }
+  ~MemoryCacheLockRecord() noexcept(false) {
+    KJ_IF_SOME(o, memoryCacheObserver) o.get()->end();
+  }
+  KJ_DISALLOW_COPY_AND_MOVE(MemoryCacheLockRecord);
+
+private:
+  kj::Maybe<kj::Own<MemoryCacheObserver>> memoryCacheObserver;
+};
+
 // Observes a specific request to a specific worker. Also observes outgoing subrequests.
 //
 // Observing anything is optional. Default implementations of all methods observe nothing.
@@ -108,6 +139,10 @@ class RequestObserver: public kj::Refcounted {
   virtual uint64_t clockRead() {
     return 0;
   }
+
+  virtual kj::Maybe<kj::Own<MemoryCacheObserver>> getMemoryCacheObserver() {
+    return kj::none;
+  };
 };
 
 class IsolateObserver: public kj::AtomicRefcounted, public jsg::IsolateObserver {